Drop characteristic measurement

ABSTRACT

An inkjet printing system with a droplet measurement apparatus is described herein. The droplet measurement apparatus has a light source with a collimating optical system, an imaging device disposed along an optical path of the collimating optical system, and a droplet illumination zone in the optical path of the collimating optical system, the droplet illumination zone having a varying droplet illumination location, wherein the light source, the imaging device, or both are adjustable to place a focal plane of the imaging device at the droplet illumination location. The droplet measurement apparatus is structured to accommodate at least a portion of a dispenser of the printing system within the droplet illumination zone.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 17/657,657, filed Apr. 1, 2022, which is a continuation of U.S.patent application Ser. No. 16/719,666, filed Dec. 18, 2019, now U.S.Pat. No. 11,318,738, issued on May 3, 2022, which claims the benefit ofU.S. Provisional Patent Application No. 62/783,767, filed Dec. 21, 2018,and U.S. Provisional Patent Application No. 62/810,481, filed Feb. 26,2019, each of which is incorporated herein by reference.

FIELD

Embodiments of the present invention generally relate to inkjetprinters. Specifically, methods and apparatus for monitoring and controlof print materials during deposition processes are disclosed.

BACKGROUND

Inkjet printing is common, both in office and home printers and inindustrial scale printers used for fabricating displays, printing largescale written materials, adding material to manufactured articles suchas PCB's, and constructing biological articles such as tissues. Mostcommercial and industrial inkjet printers, and some consumer printers,use piezoelectric dispensers to apply print material to a substrate. Apiezoelectric material is arranged adjacent to a print materialreservoir. Applying a voltage to the piezoelectric material causes it todeform in a way that applies a compressive force to the print materialreservoir, which is constructed in turn to eject print material when thecompressive force is applied.

Some inkjet printing applications rely on extreme precision inpositioning of print nozzles, quantity and type of print materialejected, and velocity and trajectory of droplet ejection. When nozzlesfail to eject print material on demand, with the correct volume,velocity, and trajectory, printing faults result and time and money mustbe spent correcting the faults. Optical systems are routinely used tomonitor droplet size and flight from print nozzles to substrates. Suchsystems typically rely on illuminating droplets of print material todetermine droplet characteristics. The droplets are typically verysmall, for example 10-15 μm in diameter, and sharp focus of the imagescaptured is helpful in ascertaining droplet characteristics withprecision. It is most useful, in addition, to capture the images whilethe droplets are close to the ejection nozzle. These considerations canconstrain the geometry of illumination apparatus. There is need in theart for flexible droplet illumination hardware.

SUMMARY

Embodiments described herein provide a droplet measurement apparatus,comprising a light source having a collimating optical system; animaging device disposed along an optical path of the collimating opticalsystem; and a droplet illumination zone in the optical path of thecollimating optical system, the droplet illumination zone having avarying droplet illumination location, wherein the light source, theimaging device, or both are adjustable to place a focal plane of theimaging device at the droplet illumination location.

Other embodiments described herein provide a printing system, comprisinga substrate support; and a print assembly operatively coupled to thesubstrate support, the print assembly comprising a print support; adispenser assembly movably coupled to the print support; and a dropletmeasurement apparatus coupled to the print support, the dropletmeasurement apparatus comprising a housing; a light source disposed inthe housing, the light source comprising a collimating optical system; adroplet illumination zone having a varying droplet illuminationlocation; and an imaging device disposed in the housing to receiveradiation from the droplet illumination zone, the light source, dropletillumination zone, and imaging device defining a focal plane that isadjustable to the droplet illumination location.

Other embodiments described herein provide a method, comprisingpositioning a dispenser of an inkjet printer in proximity to anillumination zone of a droplet measurement apparatus; emitting a beam ofcollimated light from a light source through the illumination zone;aligning a nozzle of the dispenser with an optical path of the beam ofcollimated light; emitting a droplet from the nozzle into theillumination zone; illuminating the droplet at an illumination locationusing the beam of collimated light to form a radiation signature of thedroplet; receiving the radiation signature at an imaging device; andadjusting a focal plane of the imaging device to the illuminationlocation.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentdisclosure can be understood in detail, a more particular description ofthe disclosure, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlyexemplary embodiments and are therefore not to be considered limiting ofits scope, may admit to other equally effective embodiments.

FIG. 1 is a top isometric view of a printing system according to oneembodiment.

FIG. 2A is a cross-sectional view of a droplet measurement apparatusaccording to another embodiment.

FIG. 2B is a close-up cross-sectional view of the droplet illuminationzone of the droplet measurement apparatus of FIG. 2A.

FIG. 3 is a cross-sectional view of a droplet measurement apparatusaccording to another embodiment.

FIG. 4 is a flow diagram summarizing a method according to anotherembodiment.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation.

DETAILED DESCRIPTION

A printing system is described herein that has a droplet measurementapparatus with flexible geometry for capturing droplets at differentfocal planes for various print ejector designs.

FIG. 1 is an isometric top view of a printing system 100 according toone embodiment. The printing system 100 has a substrate support 102mounted on a base 104. The base 104 comprises one or more solid massiveobjects that provide a stable foundation for the printing system 100. Insome cases, the base 104 is one or more granite blocks. Using a solidmassive object as the base minimizes unwanted vibration or othermovement of the printing system 100.

The substrate support 102 includes a working portion 106, a firststaging portion 108, and a second staging portion 110. The workingportion 106 is disposed between the first staging portion 108 and thesecond staging portion 110. The working portion 106 is supporteddirectly on the base 104, while each of the staging portions 108 and 110are supported by base extensions 112 attached to the base 104 andextending laterally from the base 104. The base extensions 112 may bemade of any structurally strong material, such as steel. The stabilityof the base 104 minimizes uncontrolled motion of the substrate and/orother printer components at the location where material is dispensedonto the substrate.

The substrate support 102, in particular the working portion 106, is atable that supports a substrate in a printing position in the printingsystem 100. The substrate support 102 has a supporting surface 103 thatprovides a low-friction or frictionless support to allow precisemovement and positioning of a substrate for printing. The table isrectangular, with a long dimension in a first direction 114 and a shortdimension in a second direction 116 perpendicular to the first direction114. During printing, the substrate is moved in the first direction 114.The short dimension is similar to a maximum dimension of a substrate inthe second direction 116, which is a cross-scan direction. The longdimension may be up to about 10 m, while the short dimension istypically 2-3 m. A third direction 118 is perpendicular to both thefirst direction 114 and the second direction 116.

The printing system 100 has a print assembly 120 juxtaposed with theworking portion 106. The print assembly 120 includes a dispenser support122 and a dispenser assembly 124. The dispenser support 122 comprisestwo stands 126, one on either side of the working portion 106 andaligned along the cross-scan direction. The stands 126 rise from thebase 104, and may be attached or integrally formed with the base 104.The stands 126 support a rail 117 that extends across the workingportion 106 in the second direction 116 from one stand 126 to the otherstand 126. Multiple stands may be used on each side of the workingportion 106 to support the rail 117, and multiple rails 117 may be usedto support devices that scan across the working portion 106, such asimaging devices and drying devices.

The dispenser assembly 124 is coupled to the rail 117 by a carriage 128,which includes an actuator that moves the carriage 128 along the rail117 to position the dispenser assembly 124 at a desired location in thesecond direction 116. During a print job, the substrate moves by thedispenser assembly 124 in the first direction 114, sometimes called thescanning direction, while the dispenser assembly 124 is positioned inthe second direction 116, sometimes called the cross-scan direction, byoperation of the carriage 128 to deposit material in a desired locationon the substrate. A dispenser housing 130 is coupled to the carriage128. One or more dispensers (not shown) may be disposed in the dispenserhousing 130 to dispense print material toward the working portion 106.The dispensers dispense print material toward the substrate as thesubstrate moves by the dispenser assembly 124. Each dispenser typicallyhas a plurality of ejection nozzles (not shown) at an ejection surfaceof the dispenser facing the working portion 106.

A print job may include depositing droplets of print material on asubstrate in an extremely precise manner. Droplets having dimension of10-15 μm are deposited at a target location on the substrate. The targetlocation may have dimension of 15-20 μm. The droplets are deposited byejecting droplets having the requisite dimension from the ejectionnozzles at a time, velocity, and trajectory toward a droplet depositionlocation, which is a predetermined location above the substrate supportwhere the target location of the substrate will be positioned when thedroplet arrives at the droplet deposition location. The size, ejectiontime, velocity, and trajectory of the droplets is determined to placethe droplets at the droplet deposition location when the movement of thesubstrate brings the target location to the droplet deposition location.The extreme precision of such processing requires excellent control ofdroplet size, ejection time, velocity, and trajectory from the ejectionnozzles.

In order to achieve such control of droplet properties, the output ofthe ejection nozzles is measured. The printing system 100 includes adroplet measurement apparatus 132 located near one of the stands 126.The droplet measurement apparatus 132 is an optical system that detectsthe interaction of specifically configured electromagnetic radiationwith droplets ejected from the dispensers in the dispenser housing 130to determine the ejection characteristics of the dispenser—droplet size,velocity, and trajectory—as a function of impulse input to the dispenserand characteristics of the print material. The droplet measurementapparatus 132 can be attached to the stand 126, or as shown here may besupported on the base 104 near the stand 126. The droplet measurementapparatus 132 is supported beside the working portion 106 of thesubstrate support 102 to allow the dispenser assembly 124 to access thedroplet measurement apparatus 132. The dispenser assembly 124 movesalong the rail 117 to the stand 126, or vicinity thereof, to engage withthe droplet measurement apparatus 132. Typically, a controller controlspositioning of the dispenser assembly 124 based on predeterminedlocation data for the droplet measurement apparatus 132 stored in, oraccessible to, the controller. An optional actuated platform 134 isshown here if the ability to raise and lower the droplet measurementapparatus 132 is desired. Such ability may be useful to move the dropletmeasurement apparatus 132 away from the dispenser housing 130, or otherequipment, when not in use. The actuated platform 134 may also be usefulto precisely position the droplet measurement apparatus 132 with respectto the dispenser housing 130 for best results in recording dropletcharacteristics.

FIG. 2A is a cross-sectional view of a droplet measurement apparatus 200according to one embodiment. The droplet measurement apparatus 200 maybe used as the droplet measurement apparatus 132 in the printing system100. The section plane of FIG. 2A is perpendicular to the firstdirection 114 of FIG. 1 . A frame 202 holds a light source 206, adroplet illumination zone 208, and an imaging device 210 in opticalengagement. The light source 206 is a collimated light source with alight source 207 and a collimating optical system 209 optically coupledto the light emitter. The light source 206 emits collimated light towardthe droplet illumination zone 208. The dispenser housing 130, withdispensers 216, are shown installed in a dispenser tray 218 in positionto engage the droplet measurement apparatus 200. A droplet is ejectedfrom an ejection nozzle (not shown) at an ejection surface 217, whichmay be a surface of an ejection plate, of a dispenser 216 toward thedroplet illumination zone 208 and into receptacle 214.

The collimated light is steered by a first steering optic 222 and asecond steering optic 224 through a droplet measurement zone 212 of thedroplet illumination zone 208 to interact with the droplet passingthrough the droplet illumination zone 208. An optional alignment optic220 aligns the collimated light with the first steering optic 222. Eachof the first steering optic 222 and the second steering optic 224 areprisms in this case, but other optical devices, or combinations, can beused as steering optics. The alignment optic 220 is also a prism in thiscase. The steering optics allow the light source 207 and the imagingdevice 210 to have optical axes that are not parallel, reducing thefootprint of the droplet measurement device 200. The steering opticsoptically couple the light source 207 and the imaging device 210 alongan optical path that proceeds through the droplet illumination zone 208.

Following interaction with the droplet, the light is steered to theimaging device 210. The imaging device 210 captures an image of thelight from the interaction with the droplet to record data about thedroplet. The imaging device 210 may be a camera, CCD array, photodiodearray, or other imaging apparatus. The imaging device is supported inoperating position by a holder 226. The holder 226 is coupled to alinear positioner 228, which in turn is coupled to a mount 230 insidethe frame 202.

FIG. 2B is a close-up view of the droplet illumination zone 208 toillustrate operation of the droplet measurement apparatus 200. The lightsource 206, droplet illumination zone 208, and imaging device 210 arepositioned along an optical path 250 of the collimated light. Theoptical path 250 is not straight in this case to allow the light source206 and imaging device 210 to be contained in a small footprint. Theejection nozzles of the dispensers 216 eject droplets along paths thatintersect with the optical path 250 at droplet illumination locations.Here, four droplet illumination locations 252A, 252B, 252C, and 252D areshown corresponding to four ejection nozzles in the ejection surface 217of the dispenser 216. The droplet illumination locations may correspondto the positions of ejection nozzles in the ejection surface 217. So, ifthe ejection surface 217 has five ejection nozzles arranged to ejectdroplets into the optical path, there will be five droplet illuminationlocations. Generally, the droplet illumination locations will be definedby the intersection of a flight path from each ejection nozzle with theoptical path 250. At each droplet illumination location 252A-D, adroplet may be illuminated by the collimated light resulting inillumination signature propagated along the optical path 250 to theimaging device 210. The light is collimated to a change in spot size ofless than about 1% per meter. Here, the optical path 250 is about 0.25meters long.

In order to image the droplet illumination and recover usable data ondroplet size, velocity, and trajectory, the imaging device 210 focusesthe light received from the droplet illumination zone 208 along theoptical path 250. Focusing the light locates a focal plane of theimagining device 210 at the droplet illumination location 252A, 252B,252C, or 252D, depending on where the droplet is to be ejected. Thus,the droplet measurement device 200 has an adjustable focal plane.Because the optics of the imaging device 210 have high resolution and/oroptical power, the focal depth or working range of the imaging device210 is small. For that reason, the imaging device 210 is movable withrespect to the frame 202. In alternative embodiments, adjusting thefocal plane of the imaging device can include adjusting a focuscomponent of the imaging device, such as a lens, mirror, or prism.

Referring again to FIG. 2A, the linear positioner 228 is actuated toextend or retract, moving the imaging device 210 toward or away from theillumination zone 208. Moving the imaging device 210 changes the lengthof the optical path 250, along with the focal limits of the imagingdevice 210 such that the working range of the imaging device 210includes the droplet illumination location 252A, 252B, 252C, or 252D, ofinterest. Thus, if a droplet is to be ejected from a nozzle aligned withthe droplet illumination zone 252A, the linear positioner 228 isactuated to position the imaging device 210 such that the working rangeof the imaging device includes the droplet illumination location 252A,and likewise with the locations 252B, 252C, and 252D.

The prisms 222 and 224 are positioned on either side of the dropletillumination zone 208, with the receptacle 214 between them. Thus, theprisms 222 and 224 are located either side of the receptacle 214. Theprisms 222 and 224 protrude upward above the upper extent of thereceptacle 214 in order to direct light from the light source 206 alongthe optical path 250 through the droplet illumination zone 208. Here,the prisms 222 and 224 are triangular, each having one right-angle edge,all three faces being rectangular. The prisms 222 and 224 are orientedsuch that a face 227 adjacent to the right angle edge faces the dropletillumination zone 208. In this orientation, an edge 229 adjacent to theface 227 extends toward the dispenser housing 130 beyond the opticalpath 250. The upward extension of the prisms 222 and 224 locates theoptical path 250 in the illumination zone at a distance from thereceptacle 214 that allows the dispenser housing 130 to be positionedwith the optical path 250 near a flexure plate 260 of the dispenserhousing 130 such that the droplet measurement apparatus 200 captures animage of the droplet before characteristics of the droplet change as thedroplet travels through the atmosphere.

The flexure plate 260 has a plurality of channels 262. In this view, thechannels 262 extend into the plane of FIG. 2B (and 2A). The channels 262are generally located around the dispensers 216 and spaced apart adistance D that is generally similar to a distance E between the twoprisms 222 and 224. Specifically the distance E is a distance betweentop extremities of the prisms 222 and 224. The channels 262 allow thedispenser housing 130, properly positioned to register the prism 222 and224 upward protrusions with the channels 262, to approach the opticalpath 250 more closely by providing additional space to accommodate theupward protrusions of the prisms 222 and 224. The channels 262 aregenerally located between the dispensers 216 such that a portion of anydispenser 216 can be positioned between the prisms 222 and 224, bypositioning the upward protrusions of the prisms 222 and 224 in thechannels 262 on either side of the desired dispenser 216. In this way,the flexure plate 260 can be positioned as close as possible to theoptical path 250 for best imaging results.

Additionally, the optical path 250 is configured to bring the opticalpath as close to the dispensers 216 as possible in the dropletillumination zone 208. Thus, the optical path 250 is brought as close aspossible to the face 227 that faces across the droplet illumination zone208 so that the light path through the prisms 222 and 224 is near thetop of the upward projections of the prisms. Such an arrangementprovides closer engagement of the ejection surface of the dispensers 216to the optical path 250.

FIG. 3 is a cross-sectional view of a droplet measurement apparatus 300according to another embodiment. The droplet measurement apparatus 300is similar in many respects to the droplet measurement apparatus 200 ofFIGS. 2A and 2B, and the same elements are labeled with the samereference numerals. The chief difference between the droplet measurementapparatus 300 and the droplet measurement apparatus 200 is that thelight source 206 and the imaging device 210 are aligned. The receptacle214 is disposed in a support 302 that extends outward to support thelight source 206 and the imaging device 210 in a transverse positionrelative to the droplet illumination zone 208. The light source 206 ispositioned on one side of the droplet illumination zone 208, and theimaging device 210 is positioned opposite from the light source 206 withthe droplet illumination zone 208 between the light source 206 and theimaging device 210. The optical axes of the light source 206 and theimaging device 210 are aligned here along an axis 310 that extends fromthe light source 206, through the droplet illumination zone 208,directly to the imaging device 210.

In this case, the imaging device 210 is coupled to a linear positioner304 that is supported on the support 302. The linear positioner 304moves the imaging device 210 closer to or further from the dropletillumination zone 208 in a transverse direction, as shown by the arrow308, to move the focal plane of the imaging device to coincide with oneof the droplet illumination locations 252 where a droplet is to bemeasured.

Positioning the light source 206 and the imaging device 210 in line canpresent challenges in getting a high resolution image of some dropletlocations. In FIG. 3 , a print assembly 312 has three dispensers, afirst dispenser 320, a second dispenser 322, and a third dispenser 324,coupled into a frame 314, which may be part of a dispenser housing likethe housing 130 of FIG. 1 . The three dispensers 320, 322, and 324 arealigned along the optical axis 310 with a spacing set by operationalspecifications of the printing system. The dispensers are shownpositioned between the light source 206 and the imaging device 210 inorder to get an image of droplets 306 immediately upon exiting thedispenser before any characteristics of the droplet change appreciably.Positioning the dispenser between the light source 206 and the imagingdevice 210 limits how close the imaging device can be positioned to somedroplet illumination locations, for example those furthest from theimaging device 210 and closest to the light source 206. In some cases,the desired image resolution and working range may be difficult toharmonize merely by moving the imaging device 210.

To augment the working range of the imaging device 210, a tunable lens330 may be coupled to the imaging device 210. A tunable lens can extendthe working range over which the imaging device 210 can deliver highresolution images, so the imaging device 210 can capture high resolutionimages of the closest droplet illumination location of the firstdispenser 320 and the furthest droplet illumination location of thethird dispenser 324.

As shown in FIG. 3 , in order to position the third dispenser 324 overthe receptacle 214 to deliver a droplet, the dispenser assembly must bemoved to the right by a distance A which is larger than a clearance Bbetween the dispenser assembly 312 and the imaging device 210. Theimaging device 210 must therefore be moved to the right by actuating thelinear positioner 304, increasing the distance between the imagingdevice 210 and the droplet illumination zone 208. If the imaging device210, without augmentation by a tunable lens, does not have sufficientworking range to deliver high resolution images at the increaseddistance, the tunable lens 330 can be used to increase the working rangeof the imaging device 210. In this way, suitable images can be capturedof droplets ejected from all three of the dispensers 320, 322, and 324.It should be noted that in any of the embodiments described herein, atunable lens such as the tunable lens 330 may be used along with, orinstead of, the linear positioner 304 or 228.

The droplet measurement apparatus 300 of FIG. 3 has, in some respects, asimpler configuration than the droplet measurement apparatus 200 ofFIGS. 2A and 2B. The droplet measurement apparatus 200, however, engagesthe dispensers 216 with the optical path 250 with minimal insertion ofthe dispensers 216 into the imaging apparatus. In the apparatus 300, theentire dispenser housing 130 is inserted between the light source 206and the imaging device 210, while with the droplet measurement apparatus200, only the end of one dispenser 216 is inserted between the uppertips of the prisms 222 and 224, with less vertical movement needed. Themost useful design can be tailored to the individual application.

FIG. 4 is a flow diagram summarizing a method 400 according to anotherembodiment. The method 400 is a method of obtaining a measurement of acharacteristic of a droplet ejected from a print material dispenser of aprinting system such as an inkjet printer. The method 400 can bepracticed using the apparatus described herein. At 402, the printmaterial dispenser is positioned adjacent to a droplet measurementapparatus of the printing system. If more than one dispenser is used,the dispensers may be housed in a dispenser assembly. The dropletmeasurement apparatus is typically located outside of a substrateprocessing area of the printing system such that the operation ofperforming droplet measurement can be done without impacting thesubstrate processing area or any substrate that might be positionedthereon.

The droplet measurement apparatus generally has a light source forilluminating a droplet in a droplet illumination zone and an imagingdevice for receiving light from the droplet illumination zone after thelight interacts with a droplet. The light source includes collimatingoptics, and the droplet measurement apparatus includes opticalcomponents to direct the light from the light source on an optical paththrough the droplet illumination zone to the imaging device.

The imaging device has a variable focal plane. In one instance, theimaging device is disposed on a linear positioner that can move theimaging device to position the focal plane of the imaging device at adroplet illumination location in the droplet illumination zone such thatan image of the droplet is sharply focused. The focal plane of theimaging device is variable because droplets may be dispensed fromnozzles at various locations in an ejection face of the dispenser, andin the case where multiple dispensers are used, the dispenser assemblymay need to move to bring a target dispenser within the working range ofthe imaging device. In another instance, the imaging device may have atunable lens with dynamically adjustable focal length to position thefocal plane of the imaging device at different droplet illuminationlocations within the droplet illumination zone. The linear positionermay also be combined with the tunable lens to further extend the workingrange of the imaging device if desired.

The droplet illumination zone is defined by the ejection face of thedispenser, and a droplet receptacle positioned in the dropletmeasurement apparatus. The ejection face of the dispenser may have aplurality of ejection nozzles distributed across the ejection face. Thereceptacle is typically sized to accommodate the areal coverage of allejection nozzles of a dispenser so that all ejection nozzles of adispenser can be fired to measure droplet characteristics without havingto move the dispenser. In the event multiple dispensers are used in adispenser assembly, the dispenser assembly may be moved to position eachdispenser at the droplet illumination zone in turn.

At 404, the print material dispenser is positioned at a test position.The test position is typically a position wherein the ejection face ofthe dispenser is located at a minimum distance from the optical path ofthe light used to illuminate droplets. In some cases the minimumdistance is less than 2 mm, for example less than 1 mm. If opticalcomponents are used to bring the light into the droplet illuminationzone, the ejection face of the dispenser, or the frame or ejection faceof the dispenser assembly, may be shaped to accommodate positioning thedispenser or dispenser assembly at the test position. For example, ifprisms are used to steer the light through the droplet illuminationzone, in order to position a dispenser at the test position, thedispenser may be positioned partly between the prisms. In such cases, ifa portion of the prisms extends above a boundary of the optical path,that is to say beyond a spatial extent of the light field used toilluminate the droplets, the ejection surface of the dispenser or thedispenser assembly may be provided with structures to accommodate theprisms so that the ejection surface can be positioned at the testposition. In one instance, a dispenser assembly has a flat nozzle platewhere the ejection surface of a plurality of dispensers is disposed, andthe nozzle plate has notches to position the top points of two prismsused to steer light through the droplet illumination zone. To positionthe dispenser at the test position, the top points of the prisms areinserted into the notches such that the ejection surface of onedispenser is positioned a minimum distance from the optical path.

At 406, one or more droplets is ejected from the dispenser into thedroplet illumination zone. The droplet passes through the dropletillumination zone into the receptacle. A droplet may be ejected fromeach nozzle of the dispenser into the receptacle without moving thedispenser, if the receptacle is sized to accommodate the entire arealcoverage of ejection nozzles in the ejection surface of the dispenser.In such cases, the droplets may arrive in the droplet illumination zoneat different droplet illumination locations arising from the arealdistribution of ejection nozzles across the ejection surface of thedispenser. In such cases, the positions of the nozzles may be providedto a controller that is coupled to the dispenser assembly and to theimaging device to position the focal plane of the imaging device at thedroplet illumination location to be used for imaging a droplet. As thedroplet illumination location changes due to ejecting droplets fromdifferent nozzles, the focal plane of the imaging device is moved toimage each droplet.

At 408, the light source is activated to provide illumination in thedroplet illumination zone at the time a droplet passes through thedroplet illumination zone. The light interacts with the droplet toproduce a signature light field, which propagates to the imaging device.

At 410, the imaging device captures an image of the signature lightfield, which can then be analyzed to determine droplet characteristicssuch as volume. In some cases, the light source is pulsed to produce aplurality of images of the droplet as the droplet passes through thedroplet illumination zone. The plurality of images can be analyzed todetermine speed and trajectory of the droplet. Volume, ejection speed,and trajectory can be used to judge the performance of the individualnozzles of the dispenser. Because the focal plane of the imaging devicecan be changed to accommodate all the nozzles of a dispenser, the entiredispenser can be tested without repositioning the dispenser.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the present disclosure may be devisedwithout departing from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. A method, comprising: positioning a dispenser ofan inkjet printer in proximity to an illumination zone of a dropletmeasurement apparatus; emitting a beam of collimated light from a lightsource comprising a collimating optical system; routing the beam ofcollimated light through the illumination zone using a prism; aligning anozzle of the dispenser with an optical path of the beam of collimatedlight; emitting a droplet from the nozzle into the illumination zone;illuminating the droplet at an illumination location using the beam ofcollimated light to form a radiation signature of the droplet; receivingthe radiation signature at an imaging device; and adjusting a focalplane of the imaging device to the illumination location.
 2. The methodof claim 1, wherein adjusting the focal plane of the imaging devicecomprises moving the imaging device.
 3. The method of claim 1, whereinadjusting the focal plane of the imaging device comprises adjusting afocus component of the imaging device.
 4. The method of claim 1, furthercomprising positioning at least a portion of the dispenser within theillumination zone.
 5. The method of claim 4, wherein the prism is afirst prism, and positioning at least a portion of the dispenser withinthe illumination zone comprises positioning the portion of the dispenserbetween the first prism and a second prism, and the first and secondprisms are located on either side of the illumination zone.
 6. Themethod of claim 1, wherein adjusting the focal plane of the imagingdevice to the illumination location comprises using a linear positionercoupled to the imaging device to position the imaging device.
 7. Themethod of claim 1, wherein the imaging device includes a tunable lens,and wherein adjusting the focal plane of the imaging device to theillumination location comprises tuning the tunable lens.
 8. The methodof claim 1, wherein the illumination location varies within theillumination zone.
 9. The method of claim 1, further comprisingreceiving the droplet using a droplet receptacle.
 10. A method,comprising: positioning a dispenser of an inkjet printer in proximity toan illumination zone of a droplet measurement apparatus; emitting a beamof collimated light from a light source comprising a collimating opticalsystem; routing the beam of collimated light through the illuminationzone using a prism; aligning a nozzle of the dispenser with an opticalpath of the beam of collimated light; emitting a droplet from the nozzleinto the illumination zone; illuminating the droplet at an illuminationlocation using the beam of collimated light to form a radiationsignature of the droplet; receiving the radiation signature at animaging device; and adjusting a focal plane of the imaging device to theillumination location.
 11. The method of claim 10, wherein the prism isa first prism, and positioning at least a portion of the dispenserwithin the illumination zone comprises positioning the portion of thedispenser between the first prism and a second prism, and the first andsecond prisms are located on either side of the illumination zone. 12.The method of claim 10, wherein adjusting the focal plane of the imagingdevice to the illumination location comprises using a linear positionercoupled to the imaging device to position the imaging device.
 13. Themethod of claim 10, wherein the imaging device includes a tunable lens,and wherein adjusting the focal plane of the imaging device to theillumination location comprises tuning the tunable lens.
 14. The methodof claim 10, wherein the illumination location varies within theillumination zone.
 15. The method of claim 10, further comprisingreceiving the droplet using a droplet receptacle.
 16. The method ofclaim 10, wherein the optical path of the beam comprises a first portionand a second portion that is not parallel with the first portion. 17.The method of claim 11, wherein aligning the nozzle of the dispenserwith the optical path of the beam of collimated light comprisespositioning a portion of the dispenser between the first prism and thesecond prism.
 18. The method of claim 11, further comprising providing aplurality of channels in a face of the dispenser to accommodateextension of the prisms into the channels.
 19. A method, comprising:positioning a dispenser of an inkjet printer in proximity to anillumination zone of a droplet measurement apparatus; emitting a beam ofcollimated light from a light source comprising a collimating opticalsystem; routing the beam of collimated light through the illuminationzone using a prism; aligning a nozzle of the dispenser with an opticalpath of the beam of collimated light; emitting a droplet from the nozzleinto the illumination zone; illuminating the droplet at an illuminationlocation, defined by a position of the nozzle, using the beam ofcollimated light to form a radiation signature of the droplet; receivingthe radiation signature at an imaging device; and adjusting a focalplane of the imaging device to the illumination location.
 20. The methodof claim 19, wherein adjusting the focal plane of the imaging devicecomprises extending the working range of the imaging device to encompassthe illumination location.